Did she walk into a wall?
She sounds like a total idiot to me, I think in this case it would be in your daughters best mental health not to associate with the "other woman" personally I think you should bring it to court!
Im going through custody issues right now and its killing me so I understand the pain abit except I dont have another woman trying to play "mom" to my kids, that would send me into a rage!
Another thing to look into is seeing if you can get supervised custody when the kids visit their dad(if he was abusive.)